the current implementation for the [[SMILA/Documentation/DeltaIndexingManager | DeltaIndexingManager]] has several problems or short comings which are listed under the section [[#Ideas (under discussion)|Ideas (under discussion)]]. if the idea is rather large, an own page is usually better and should be created as a child to this page. it still should have an own section that at least must contain a link to the page..

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==== Why DeltaIndexing ====

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The initiating authors should edit only their own sections and not those of others.

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DeltaIndexing is used to speed up repeated indexing of the same DataSource. DeltaIndexing reflects the state of the DataSource in the index and can so determine if a Record of a DataSource is new, has changed or was deleted. An ID and a HASH-Token (which is build over characteristic data that shows a Record has changed, e.g. last modification date) are used to compute this information. In general as a consequence, less data has to be accessed on the DataSource and only the relevant data is indexed. For each DeltaIndexing run all Records that are new, have changed or have NOT changed are marked with a visited flag. At the end of the run, if no errors have occurred, all Records that have not been visited are computed. These records are the ones that are still in the index but are not available on the DataSource anymore (they were deleted or moved). These are also deleted from the index.

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each subsection/page should state:

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* context such as: author, data, based on SVN revision

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* motivation/problem

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* a solution proposal

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ideas that have been implemented are moved to their own page and referenced in [[#Implemented Changes|Implemented Changes]].

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==== The Problems ====

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== Ideas and Problems (under discussion) ==

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* One Problem at the moment is, that because SMILA's processing of incoming Records is asynchronous, DeltaIndexing does NOT really reflect the state of a Record in the index, as there is no guarantee that a Record is indexed after it was successfully added to the Queue. This could be achieved by implementing Notifications that update the DeltaIndexing state using this information. If this is done, then the computation of DeltaIndexing-Delete has to wait for all Queue entries to pass the workflow. This is a complex process which seems to be error-prone. Is it really necessary to reflect the index state or is it enough to reflect the last crawl state ?

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* the API of the ConnectivityManager includes parts of the API of the DeltaIndexingManager, which makes it more complex than necessary. Also it implicates that the ConnectivityManager has an internal state, as DeltaIndexing for a DataSource has to be initialized and finalized. This interfaces forces it's clients to make use of DeltaIndexing and to follow a strict workflow (initialize, add records, optionally call DeltaIndex-Delete and delete the returned IDs, finish). Even if this usage was configurable, the API is - simply spoken - ugly.

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=== DeltaIndexing reflects crawl state rather than index state ===

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=== New Ideas ===

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One Problem at the moment is, that because SMILA's processing of incoming Records is asynchronous, DeltaIndexing does NOT really reflect the state of a Record in the index, as there is no guarantee that a Record is indexed after it was successfully added to the Queue. This could be achieved by implementing Notifications that update the DeltaIndexing state using this information. If this is done, then the computation of DeltaIndexing-Delete has to wait for all Queue entries to pass the workflow. This is a complex process which seems to be error-prone. Is it really necessary to reflect the index state or is it enough to reflect the last crawl state ?

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==== Seperated Interfaces ====

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=== Extract Session Interface from DeltaIndexingManager ===

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<b>This approach was realized</b>

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I suggest to separate ConnectivityManager interface and DeltaIndexingManager interface. It makes both APIs more clear and focused. We should think about SMILA more of a "construction kit" than a "ready for all issues salvation". E.g. if someone wants to connect to SMILA, not using Crawlers or Agents but using the benefits of DeltaIndexing, all the components he needs are there. He can implement his own importer using the DeltaIndexingManager and ConnectivityManager interfaces. There is no need to provide the whole functionality "en-block". At the moment I see no urgend need for a remote interface (SCA). This could be neccessary in certain deploymnent scenarios, where the same DataSource (e.g. a website) is crawled by various Crawer/CrawlerController combinations and therefore must be handled by the same DeltaIndexingManager. But this coul'd also be achieved by the corresponding implementation itself (e.g. a DeltaIndexingManager that holds it's state in a distributed database). If we decide that an SCA interfacet is needed, it can be added easily.

// same functionality for Compound objects, remember not to overload methods when using SCA

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Notes: If calls to ConnectivityManager are NOT relevant for DeltaIndexingState (e.g. if it's enough that a call of add/delete succeeded, not the successfull adding to the Queue is required) they could forego a return value and the ConnectivityException and then in the SCA interface these methods could be annotated with @oneway to improve performance. Via callbacks it would still be possible to send back information asynchronously. But if feedback is required, the synchronous method call is much easier to use.

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==== Usage of DeltaIndexingManager ====

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# used by CrawlerController: This approach would not change much of the current programming logic. Only that the CrawlerController would communicate with two references instead of one. <b>This approach was realized</b>

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# used by Crawlers: This is a radical change as this also affects the Crawler interface. Crawlers could directly communicate with the DeltaIndexingManager and provide only those Records that pass DeltaIndexing (are new, nedd an update). CrawlerController and Crawler could implement a Consumer/Producer pattern which should improve performance. No more sending of arrays with DIInformation and thereafter retrieving the Record objects. DeltaIndexing-Delete information is computed in the Crawler and can passed to the CrawlerController as regular Records (only the ID is set) and a delete flag to notify the CrawlerController that this Record is to be deleted. This should reduce communication overhead, as the DIInformation has not to be passed between multiple components and the whole process can work multithreaded. Of course this adds a lot more logic to the Crawler and demands more knowledge from a Crawler developer. It would also mean that ID and HASH are generated in the Crawler. The downside is that each Crawler has to implement the DeltaIndexing workflow themselves. <br>We could even move all execution logic to the Crawler. CrawlerController would become obsolete. Then Crawlers would handle everything themselves - communication with DeltaIndexingManager, CoumpoundHandlers and ConnectivityManager. I think in this way the best performance can be achieved, as the setup is the very simple. No unnecessary passing of data between components. But a lot of logic has to be re-implemented in every Crawler. I wonder if there is a chance to minimize this.

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== New Feature: DeltaIndexing On/Off ==

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=== Motivation ===

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It should be possible to turn the usage of DeltaIndexing on and off, either to reduce complexity or to gain better performance.

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=== Draft ===

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A simple boolean logic (on/off) seems to simple, as I see 4 possible use cases (modes):

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* <b>FULL</b>: DeltaIndexing is fully activated. This means that

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** each Record is checked if it needs to be updated

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** for each Record an entry is made/updated in the DeltaIndexingManager

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** Delta-Delete is executed at the end of the import

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* <b>ADDITIVE</b>: as <b>FULL</b>, but Delta-Delete is not executed (we allow records in the index that do not exist anymore

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* <b>INITIAL</b>: For an initial import in an empty index or a new source in an existing index performance can be optimized by

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** NOT checking if a record needs to be updated (we know that all records are new)

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** adding an entry in the DeltaIndexingManager for each Record. This allows later imports to make use of DeltaIndexing

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** NOT: executing Delta-Delete (we know that no records are to be deleted)

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* <b>DISABLED</b>: DeltaIndexing is fully deactivated. No checks are done, no entries are created/updated, no Delta-Delete is executed. Later runs cannot benefit from DeltaIndexing

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As always, Delta-Delete MUST NOT be executed if any errors occur during import as we do not want to delete records erroneously!

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=== Configuration ===

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To configure the mode of DeltaIndexing execution, an additional parameter is needed in the IndexOrderConfiguration:

The execution logic has to be added in parts to the CrawlerController (CrawlThread) and ConnectivityManager. Therefore the mode has to be added to the ConnectivityManager interface. The problem is, that still initiialize and finish need to be called, and that the MODE then controls if and how DeltaIndexing is used. This makes the usage and implementation of ConnectivityManager more and more complex and obscure (too many special cases).

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==== Alternative Concept ====

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The execution logic has to be added either

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* to the CrawlerController (CrawlThread) only. It decides what actions to perform on the given mode.

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* to the Crawler themselves, if the more radical change is implemented

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== Alternative API ==

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For a better separation of tasks and an easy handling of locks on data sources during a delta indexing run, we could introduce the following interfaces. The implementations should only be proxies using the same DeltaIndexingManager service implementation, so that a DeltaIndexingSession may internally use another service if the initial one becomes unavailable.

For a better separation of tasks and an easy handling of locks on data sources during a delta indexing run, we could introduce the following interfaces. The implementations should only be proxies using the same DeltaIndexingManager service implementation, so that a DeltaIndexingSession may internally use another service if the initial one becomes unavailable.

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<b>This approach was not realized.</b>

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But a sessionId was introduced to distinguish between different sessions without relying on thread ids. See [https://bugs.eclipse.org/bugs/show_bug.cgi?id=279243 https://bugs.eclipse.org/bugs/show_bug.cgi?id=279243]

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==== Discussion ====

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===== modifications to the interfaces =====

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TM 2009 10 15:

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i second the notion to extract a session interface. but i also would do a few renames and changes like so:

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<source lang="java">

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public interface IDeltaIndexingManager {

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/**

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* Initializes the internal state for an import of a dataSourceID and creates a session wherein it establishes a lock

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* to avoid that the same dataSourceID is initialized multiple times concurrently. It returns an object for the session

Here is another idea based on the changes introduced with [[SMILA/Specifications/DeltaIndexingAndConnectivtyDiscussion09/Separate_Interfaces_for_ConnectivityManager_and_DeltaIndexingManager]] but taking it further that not the CrawlerController communicates with DeltaIndexingManager but each Crawler:

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This is a radical change as it also affects the Crawler interface. Crawlers could directly communicate with the DeltaIndexingManager and provide only those Records that pass DeltaIndexing (are new, nedd an update). CrawlerController and Crawler could implement a Consumer/Producer pattern which should improve performance. No more sending of arrays with DIInformation and thereafter retrieving the Record objects. DeltaIndexing-Delete information is computed in the Crawler and can passed to the CrawlerController as regular Records (only the ID is set) and a delete flag to notify the CrawlerController that this Record is to be deleted. This should reduce communication overhead, as the DIInformation has not to be passed between multiple components and the whole process can work multithreaded. Of course this adds a lot more logic to the Crawler and demands more knowledge from a Crawler developer. It would also mean that ID and HASH are generated in the Crawler. The downside is that each Crawler has to implement the DeltaIndexing workflow themselves. <br>We could even move all execution logic to the Crawler. CrawlerController would become obsolete. Then Crawlers would handle everything themselves - communication with DeltaIndexingManager, CoumpoundHandlers and ConnectivityManager. I think in this way the best performance can be achieved, as the setup is the very simple. No unnecessary passing of data between components. But a lot of logic has to be re-implemented in every Crawler. I wonder if there is a chance to minimize this.

Motivation for this page and usage

the current implementation for the DeltaIndexingManager has several problems or short comings which are listed under the section Ideas (under discussion). if the idea is rather large, an own page is usually better and should be created as a child to this page. it still should have an own section that at least must contain a link to the page..

The initiating authors should edit only their own sections and not those of others.

each subsection/page should state:

context such as: author, data, based on SVN revision

motivation/problem

a solution proposal

ideas that have been implemented are moved to their own page and referenced in Implemented Changes.

Ideas and Problems (under discussion)

DeltaIndexing reflects crawl state rather than index state

One Problem at the moment is, that because SMILA's processing of incoming Records is asynchronous, DeltaIndexing does NOT really reflect the state of a Record in the index, as there is no guarantee that a Record is indexed after it was successfully added to the Queue. This could be achieved by implementing Notifications that update the DeltaIndexing state using this information. If this is done, then the computation of DeltaIndexing-Delete has to wait for all Queue entries to pass the workflow. This is a complex process which seems to be error-prone. Is it really necessary to reflect the index state or is it enough to reflect the last crawl state ?

Extract Session Interface from DeltaIndexingManager

For a better separation of tasks and an easy handling of locks on data sources during a delta indexing run, we could introduce the following interfaces. The implementations should only be proxies using the same DeltaIndexingManager service implementation, so that a DeltaIndexingSession may internally use another service if the initial one becomes unavailable.

This is a radical change as it also affects the Crawler interface. Crawlers could directly communicate with the DeltaIndexingManager and provide only those Records that pass DeltaIndexing (are new, nedd an update). CrawlerController and Crawler could implement a Consumer/Producer pattern which should improve performance. No more sending of arrays with DIInformation and thereafter retrieving the Record objects. DeltaIndexing-Delete information is computed in the Crawler and can passed to the CrawlerController as regular Records (only the ID is set) and a delete flag to notify the CrawlerController that this Record is to be deleted. This should reduce communication overhead, as the DIInformation has not to be passed between multiple components and the whole process can work multithreaded. Of course this adds a lot more logic to the Crawler and demands more knowledge from a Crawler developer. It would also mean that ID and HASH are generated in the Crawler. The downside is that each Crawler has to implement the DeltaIndexing workflow themselves. We could even move all execution logic to the Crawler. CrawlerController would become obsolete. Then Crawlers would handle everything themselves - communication with DeltaIndexingManager, CoumpoundHandlers and ConnectivityManager. I think in this way the best performance can be achieved, as the setup is the very simple. No unnecessary passing of data between components. But a lot of logic has to be re-implemented in every Crawler. I wonder if there is a chance to minimize this.